Patrick Thomson patrickt

A Minimally Painful Guide to Monad Transformers

Why use monad transfomers? Well, we can take code that looks like this:

contentTypesProvided = do
  macM <- HM.lookup "mac" <$> params
  case macM of
 (Just aMac) -&gt; do

	{-# LANGUAGE GADTs, InstanceSigs, MultiParamTypeClasses, TypeApplications, TypeFamilies, TupleSections #-}

	module Moore where

	import Control.Comonad
	import Control.Monad.Zip
	import Control.Monad.Reader
	import Data.Copointed
	import Data.Distributive
	import Data.Machine.Plan

	{-# LANGUAGE GADTs, LambdaCase #-}

	-- I haven't compiled this and wrote it before any coffee
	-- so please don't laugh at me if it doesn't compile lol

	module Turtle ( Turtle, left, right, forward, backward) where

	import Graphics.Gloss
	import Control.Monad.State
	import Control.Lens

	# Append to a history file (~/.zhistory)
	setopt appendhistory
	# Auto-cd to directories if the only input provided is a directory.
	setopt autocd
	# Auto-expand substitution strings in the prompt
	setopt prompt_subst
	# Enable auto push/pop tracking, a la a browser
	setopt auto_pushd

	# Set up the vcs_info module to work with git, svn, and git-svn

	#!/usr/bin/env stack
	-- stack --resolver=lts-8.2 runghc --package text
	{-# LANGUAGE OverloadedStrings #-}

	-- example usage:
	-- $ clapper "I wrote this to troll @peterb"
	-- yields
	-- I 👏 wrote 👏 this 👏 to 👏 troll 👏 peterb 👏

	import Data.List (intersperse)

	GNU gdb (GDB) 7.6.1
	Copyright (C) 2013 Free Software Foundation, Inc.
	License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
	This is free software: you are free to change and redistribute it.
	There is NO WARRANTY, to the extent permitted by law. Type "show copying"
	and "show warranty" for details.
	This GDB was configured as "x86_64-sun-solaris2.11".
	For bug reporting instructions, please see:
	<http://www.gnu.org/software/gdb/bugs/>...
	Reading symbols from /root/snapboard-5c24579/snapboard...done.

	execve("snapboard", 0xFFFFFD7FFFDFFB78, 0xFFFFFD7FFFDFFB98) argc = 3
	sysinfo(SI_MACHINE, "i86pc", 257) = 6
	mmap(0x00000000, 56, PROT_READ\|PROT_WRITE\|PROT_EXEC, MAP_PRIVATE\|MAP_ANON, 4294967295, 0) = 0xFFFFFD7FFF390000
	mmap(0x00000000, 4096, PROT_READ\|PROT_WRITE\|PROT_EXEC, MAP_PRIVATE\|MAP_ANON, 4294967295, 0) = 0xFFFFFD7FFF380000
	mmap(0x00000000, 4096, PROT_READ\|PROT_WRITE, MAP_PRIVATE\|MAP_ANON, 4294967295, 0) = 0xFFFFFD7FFF370000
	mmap(0x00000000, 4096, PROT_READ\|PROT_WRITE, MAP_PRIVATE\|MAP_ANON, 4294967295, 0) = 0xFFFFFD7FFF360000
	memcntl(0xFFFFFD7FFF398000, 96992, MC_ADVISE, MADV_WILLNEED, 0, 0) = 0
	mmap(0x00000000, 4096, PROT_READ\|PROT_WRITE\|PROT_EXEC, MAP_PRIVATE\|MAP_ANON, 4294967295, 0) = 0xFFFFFD7FFF350000
	memcntl(0x00400000, 16885696, MC_ADVISE, MADV_WILLNEED, 0, 0) = 0
	resolvepath("/usr/lib/amd64/ld.so.1", "/lib/amd64/ld.so.1", 1023) = 18